1. Field of the Invention
The subject invention relates to structures exhibiting improved transmission of electromagnetic radiation in the radar wave region of the spectrum, and to structural materials which allow the construction of such structures.
2. Description of the Related Art
Innumerable technological improvements in the amplification, signal conditioning and treatment, radiation and reception of electromagnetic radiation in the radar wave portion of the spectrum have been made since the inception of the use of radar in the 1930's, and extension of the range of operable frequencies has been made well into the Ghz region. However, because most radar antennae are enclosed, transmission of radar waves in the vicinity of the antenna is still problematic.
The enclosure surrounding a radar antenna, regardless of its actual shape, is termed a radome. Radomes are strong, electrically transparent shells which provide protection of the antenna from meterological events, especially wind and water. In the case of military radar, protection from concussive effects of nearby guns or the blast from near hits is also required. Some protection from ballistic energy is also required.
Radomes vary in size and shape from simple conical or parabolic housings whose diameters are measured in centimeters, to large dome shaped structures tens of meters in diameter. The construction methods and structural materials utilized in building radomes are equally varied.
Ideally, the principle radome material should have the same transmission properties as air. However, this ideal cannot be achieved, and considerable losses in signal strength and changes in the wave envelope occur because of the electrical characteristics of the structural materials.
Due to large differences between the dielectric constants of the structural materials and air, reflections occur at the air/material interfaces, causing signal loss as well as complicating signal processing. In addition, due to the differences in geometric shape of the antenna and its radome, the various signal paths are generally not equal and thus refractance of the signal also occurs. Finally, the construction materials exhibit a power loss through absorption of the signal. This absorption, quantified by the loss tangent, is roughly analogous to the phenomenon of electrical resistance in the transmission of current electricity, causes heating of the radome material, and is the basis for dielectric heating so commonly used in industry.
Just as low dielectric constant and loss tangent are desirable for use in radomes for the purpose of transmitting and receiving radar waves with minimum signal attenuation and reflectivity, these same attributes are also of use in the design and construction of low observable, or "stealth" structures. In such structures, minimum reflectance of radar waves from the surface is desirable, but coupled with high absorbtion of the radar waves within the structure. For such applications, a surface of low dielectric constant is required. The interior of such structures should exhibit a loss tangent which enables rapid attenuation of radar frequencies. This loss tangent must be tailored for the specific application, often in many fine gradations.
When radomes are constructed from fiber reinforced composites, epoxy resins and bismaleimide matrix resins are generally used due to their excellent physical characteristics. Unfortunately, the electrical characteristics of these materials are far from ideal. The fiber reinforcement in such applications generally consists of fibers spun from fused quartz, as these fibers have dielectric constants and loss tangents far better than ordinary glass fibers formed from borosilicate glasses.
When radomes are constructed from honeycomb material, especially common for large radomes, the outer, face-plies are generally a thin fiber reinforced composite prepared from epoxy or bismaleimide impregnated heat-curable prepregs, while the honeycomb itself may be prepared from similar prepregs, from phenolic resin impregnated prepregs, or from extruded thermoplastics such as high temperature service polycarbonates or polyimides. In this case, as with traditional fiber-reinforced composites, the resin systems utilized for forming the face plies and the honeycomb often do not have the desired electrical characteristics. Moreover, the face sheets are adhesively joined to the honeycomb core through the use of film adhesives. In the past epoxy, bismaleimide, and phenolic film adhesives have been used, and thus the film adhesives suffer from the same electrical drawbacks as the matrix resins used in the face plies. Moreover, many of these adhesives have less than the desired ability to bond to certain prepregging materials, particularly those prepared using bismaleimide matrix resins.
Ceramic materials have been utilized for small radomes, particularly for missle applications. However it is well known that ceramic materials tend to be brittle and difficult to fabricate. When adhesives are utilized to bond ceramic constructs to themselves, to other parts of the radome structure, or to the missle or other base, once again epoxy and other common adhesives have been used, adhesives which have higher dielectric constants and greater loss than the ceramic materials they join.
Sintered polytetrafluoroethylene (PTFE) powders and fibers have been used in radomes due to their excellent electrical properties, as disclosed in U.S. Pat. Nos. 4,364,884 and 4,615,859. However, such structures are difficult to fabricate and lack the strength required for many military applications. PTFE fibers could be used in conjunction with epoxy or bismaleimide matrix resins, but would then suffer from the electrical disadvantages of these resins.
In U.S. Pat. No. 4,436,569, a protective cover for use with radomes or other aircraft structures is proposed in which a polyethylene/polyurethane composite is adhesively bonded to the underlying structure, preferably with a polyurethane adhesive. Unfortunately, the polyurethane polymer and adhesive have relatively low strength properties at elevated temperatures, as does also the polyethylene.
Bismaleimide-triazine resins have been proposed for use in electrical circuit boards by the Mitsubishi Gas Chemical Company, Inc., in their brochure entitled "BT Resin". These resins contain difunctional monomers having a bismaleimide group as one of the functional groups, and a cyanate group as the other. However the reported dielectric constant is reported to be high, being greater than 4.2 at 1 Mhz. Thus these resins would not appear to have the low dielectric constant desired of a prepregging resin or adhesive based on this publication, and moreover, their electrical behavior in the radar region (&gt;100 Mhz), is unknown.
In U.S. Pat. No. 4,353,769, a composite material for radomes is proposed in which Astroquartz.RTM. fiber reinforcing fabric is impregnated with a specific prepolymer made from ethyleneglycol, 4,4'-methylenediphenylenediisocyanate, and 2,4-toluenediisocyanate. However the dielectric constants of these materials are still higher than desirable, and loss tangents are truly improved over only a narrow compositional range. Moreover, the cured prepreg lacks adequate high temperature performance due to the use of polyurethane as the matrix resin.
The use of high temperature polimides has been proposed for fiber reinforced radomes in supersonic applications. See, for example, M. C. Cray, "High Performance Radome Manufacture Using Polyimides," Vol. 1, p. 309-319, Proceedings, International Conference on Electromagnetic Windows, 3d. (1976), and T. Cook, "Supersonic Radomes in Composite Materials," Vol. 1, p. 4-1 to 4-14, Proceedings of the Third Technology Conference (1983). However thermosetting polyimides are difficult to process, especially with regard to the formation of volatiles during cure, and thermoplastic polyimides require high temperature extrusion or pressure forming, which again renders their use problematic. Furthermore, it is difficult to formulate suitable adhesives from polyimides, particularly when the adherends are composites prepared from bismaleimide resin impregnated prepregs.
E-glass reinforced PTFE and S-glass reinforced perfluoroepoxy resins have been proposed as candidates for radome applications by E. A. Welsh, "Evaluation of Ablative Materials for High Performance Radome Applications," Symposium on Electromagnetic Windows, 15th, p. 179-185, (1980). Reinforced PTFE is expensive and difficult to process, however; and perfluoroepoxy resins are both difficult to prepare as well as not being readily available.
The use of a variety of thermoplastics including polyimides, polyamide-imides, polyphenylene sulfides, nylons, polyesters, and polyethersulfones, among them, has been proposed by R. A. Mayor in "Cost Effective High Performance Plastics for Millimeter Wave Radome Applications," Proceedings, Twenty-Fourth National SAMPE Symposium, Book 2, p. 1567-1591 (1979). However many of these materials, such as melt processable nylons and polyesters do not have the high temperature capabilities desired, and the high performance thermoplastics such as the polyimides and polyethersulfones are difficult to process. In addition, many of these thermoplastics have undesirably high dielectric constants and loss tangents.
In U.S. Pat. 4,568,603 is disclosed a fiber reinforced syntactic foam useful for lightweight structures such as microwave waveguides. However, as can be surmised from their intended use, these materials are microwave reflective rather than transparent. The use of epoxy resins in formulating such syntactic foams and the inclusion of graphitic or carbon fibers is in agreement with this conclusion. Thus the use of such syntactic foams as adhesives, fillers, or as structural materials in radar applications requiring transparency, is prohibited.
Thus there exists a need for structural materials, particularly structural adhesives, which have low dielectric constants and low loss tangents in the radar region of the spectrum, and which also have superior strength, toughness, and adhesive qualities. Thus far such products have not been available to the industry.